1. Field of the Invention
This invention relates in general to the assembly of structural components, and specifically to the application of a resin layer or skin on the surface of a frame member.
2. Description of the Prior Art
In prior art methods, the assembly of an understructure of an aircraft is achieved by applying liquid resin to the surface of various components making up the aircraft's understructure to form a shim. The liquid resin is applied by hand directly to the aircraft understructure and is allowed to cure. Once the resin has cured, the entire understructure assembly is sent to a precision five-axis mill where the liquid resin is machined to precise profile tolerances which match the inner profile of the fuselage panels. Once machined, the fuselage panels are attached on top of the cured resin layer, which is often referred to as a shim.
There are many disadvantages to these methods. First, applying adequate liquid resin coverage to the surface of the understructure is highly important. Applying the resin by hand, however, often results in uneven resin coverage. Since the liquid resin forms a skin, or amine carbonate, when exposed to air, it is important to apply enough material to achieve the desired thickness the first time the resin is applied. If adequate resin is not applied to the understructure, the resin must be reworked. This process, however, is messy and labor-intensive, and the added resin may not develop the needed bond strength. In addition, if the thin areas of resin coverage are not discovered until after the cured resin has been machined, it is likely the resin will require an additional machining, which requires additional resin and will lead to longer construction times and higher costs.
The second disadvantage to the prior art method is that hand application often results in too much resin being applied. Applying too much resin results in wasted material, which is expensive. In addition, this requires more machining time because an increased number of machining passes is necessary to remove the unneeded resin which, again, adds unnecessary cost to the assembly process.
The third disadvantage to the prior art method is that hand application involves a high risk of air entrapment. Since the resin material is applied wet using a wooden tongue depressor, there is always a risk of creating air pockets while spreading the resin on the understructure surface. These air pockets reduce the structural properties of the cured resin and can be exposed later during the machining process requiring additional, and often difficult, repairs.
The fourth disadvantage to the prior art method is that it is difficult to apply the liquid resin in a vertical, or similar, attitude. Due to the viscosity of the liquid resin material in the wet uncured state, it is difficult, if not impossible, to apply the shim to an overhead structure because “slumping” of the material will occur. This results in an unacceptable surface which will require additional and costly reworking.
In addition, in prior art methods when the liquid resin is exposed to air during curing, there is an amine carbonate reaction caused by moisture in the air that causes the resin to form a skin. If this reaction occurs, the top surface of the cured resin must be removed using a laborious sanding process. Moreover, once the resin has been applied, the understructure must sit undisturbed for a minimum of five hours. During this time, the resin will be allowed to cure to a hardness capable of being handled. If, however, the understructure is disturbed during this time, there is a risk of disturbing the resin which will, once more, require costly manual reworking.
In view of the foregoing, the inventors of the present invention have realized the need for a less expensive, more efficient, and dependable method of understructure assembly.